Michael Fossum and Ronald Garan will prep the Japanese space lab for installation on the international space station. Later, astronauts working from inside will use a robot arm to move the lab from the shuttle and anchor it.
Spacewalkers also will try out some cleaning methods on a jammed solar rotating joint that has hampered energy production at the space station since last fall.
Meanwhile, NASA is investigating the worst launch pad damage in 27 years of space shuttle flight.
A large section of the flame trench broke apart Saturday, sending chunks of the large heat-resistant bricks and concrete mortar past a chain-link fence more than 500 meters away.
None of the debris appeared to hit Discovery.